Method and apparatus for determining the properties of acoustic materials



Jan. 14, 1930. E. c. WENTE 1,743,414 METHOD AND APPARATUS FOR DETERMINING THE PROPERTIES OF ACOUSTIC MATERIALS Filed July 15, 1926 lnvernbr:

' Edward C. Wenfe by W 411}.

Patented Jan. 14, 1930 UNITED STATES PATENT OFFICE ED'WARIII) C. WEN TE, OF NEW YORK, N. Y., ASSIGNORTO WESTERN ELECTRIC COMPANY,

INCORPORATED, 0F NEW YORII, N.

METHOD AND APPARATUS FOR- Application filed m 13,

material under test and on the other end of which sound waves are impressed. In particular, the amplitude coefficient of reflection is determined from the ratio of the maximum to minimum pressure at the input end for varying lengths of the column.

The invention is embodied in a tube having walls practically impervious to sound, a tight- -fitting plunger having a solid end, and arranged-t0 mount the material under test for closlng one end of the tube, an electromagnetic receiver for impressing waves on the other end, and an instrument for measuring the pressure within the tube at the" face of the diaphragm of the receiver.

The invention may be more readily understood by reference to the following detailed description in" connection with the drawing in which, Fig. 1 shows a simple diagram for illustrating the theory of the method of this invention, and- Fig. 2 shows one embodiment of an apparatus according to this invention.

Referring to Fig. 1 there is shown a tube 5 closed at one end C and arranged to receive sound waves at the other end A. Theportion of the tube B-C is filled 'with a material, the absorption coefficient of which it is desired to determine. It is assumed that the lZa lateral 'walls of the tube are impervious to Y., A CORPORATION OF NEW YORK DETERMINING THE PROPERTIES OF ACOUSTIC MATERIALS 1926. Serial No. 122,085.

Where, Z and P are, respectively, the characteristic impedance and propagation constant of the transmission medium AB, Z is the length of the medium A-B and Z is the acoustic impedance at B. This expression is obtained by reference to Equation 61, v

page 87 of The Propagation of Electrical 0 Currents in Telephone and Telegraph Conductors, J. A. Fleming, Van Nostrand Company.

Z may be expressed as a complex quantity,

If the medium in the tube A-B is air it may be assumed that there is no dissipation and therefore:

The absolute value of impedance is:

a. b 26R tan R tan? fil R 26R tan 61+ (a? 6 tan 3L As Z is varied I Z, I passes through maximum and minimum values. Differentiating Equation (6) and equating to zero, to obtain these values we get,

tan Bl.='m=k W1 (7) One of the signs in Equation (7) corresponds to a maximumand the other to a minimum value of \Z,\.

Substituting these values for tan Bl in Equation (6), and simplifying, we get,

where,

Theinegative sign may be shown to be inadmissible, therefore solving for b we get,

By analogy'from Equation (54) Chapter XI of Transmission Circuits for Telephonic Communication, by K. S. Johnson, D. Van

Nostrand Company, New York, it follows .that, where I I. and I, represent,respectively, the incident wave, the absorbed wave, and the reflected wave at B,

v Therefore, since the incident power at B, v

v W is equal to the sum of the absorbed power,

' sorption of power,

W., and the reflected power Wt,

from which I as the amplitude coefi. e., the ratio of the absorbed to' the incident power, is equal to The absolute value of C is,

a m u-a b (13) D, the coefficient of ab- Letting, p

R a b 2aR =K2. we get, 72:1. 01: Flu

From Equations (8) and (14) 'R aK 6m (16) R bm K T Hence, substituting (17) in Equation (8) Substituting the value of b from- Equation (11) in Equation (19),.

I AK4=SI2 1 Or since S= i ijfrom Equation (11),

Substituting this value in Equation 15 we get,

This gives the absolute'value of the coeffieient of reflection in terms of the ratio of the maximum to the minimum values of the impedance at A.

If the'waves are lmpressed on the tube at If the phase angle of the reflection coefficient is also desired the distance Z from A to surface of the material under test) should be measured for either a maximum or minimum lmpedance. From'that quantity and the ratio A both a and b may be calculated by B (i. e., in the apparatus of Fig. 2, the distance. from the face of the diaphragm to the the use of Equations (5), (7), (8) and (10).

The characteristic impedance and propagation constant of the material itself may be determined by a method analogous to that employed for electrical networks by measuring the ratio and product of the impedances at B when the tube is-closed at C and terminated in a tube of one quarter wave length. These impedances correspond, respectively, to the open circuit and short circuit impedances of an electrical structure.

Referring to Fig. 2,'there isshown one embodiment of an apparatus for carrying out the method of this invention: This apparatus comprises a tube 6 having walls practically impervious to sound and having a suflicient inner diameter to minimize internal friction. A tube of A steel having an inside diameter of 3 was found suitable for this purpose. -An electro-dynamic receiver 7 is arranged so that its diaphragm 8 completely closes one end of the tube. This receiver comprises a-magnetic core 9 having an annular air-gap 11 and a magnetizing winding 10. A coil 12 attached to the outer rim of the diaphragm 8 is arranged to vibrate within this air-gap. The diaphragm.

8 is composed of solid metal and is mounted on the inner surface of the tube by means'of a ring 13 of chamois. This mounting gives a stiflness reactance which is negligible in comparison with the mass reactance of the 1 diaphragm so that the velocity of the diaphragm is practically independent of the impedance into which it works. The diaphragm completely fills the tube and vibrates with a plunger action setting up plane waves. The tube 6 is closed at the other end by means of a snugly fitting piston 14:. This piston has a solid back 15 and is made hollow so that the material 16, under consideration may be mounted within it. The piston is also provided with a rod 17 formoving it within the tube 6 so as to give the desired length of air column. A small tube 18 is sealed into the large tube so that its end is as near as possible to the face of the diaphragm 8. The outer end of this tube is sealed into a condenser transmitter 19 whose diaphragm vibrates in accordance with the pressure wave generated at the other end of the tube 18 and with an amplitude of vibration depending upon the position of the piston 14 in the tube 6. The electrical terminals of the transmitter 19 are connected to the input. of a vacuum tube amplifier 20 having output terminals connected to a potentiometer 21 and a voltmeter 22. Electrical waves of varying frequencies are impressed on the moving coil 12 of receiver 7 from a variablefrequency source 23.

The operation of the apparatus is as follows: The material 16 under test is mounted in the piston 14. A wave of frequency at which'the characteristics of the material are desired is impressed upon the receiver 7 to impress sound waves upon the air in the tube 6. The piston is then moved to give-maximum and minimum readings on the potentiometer 21 and voltmeter 22. The ratio of these maximum and minimum readings is determined and the amplitude coefficient of reflection, C is calculated therefrom by means of. the Formula (22). The test may be repeated for waves of other frequencies to determine the characteristics of the material over a range of frequencies.

What is claimed is:

1. The method of determining the acoustic properties of materials which comprises measuring the pressure at the input end of an acoustic line having its end terminated by means of the material under investigation then varying the length of the same line and again measuring the pressure at the same point.

2. The method of measuring the acoustic properties of materials which comprises measuring the maximum and minimum input impedance of a variable length acoustic line having its' output end terminated by means of the material under investigation.

3. The method of measuring the acoustic properties of materials by means of a varia-.

ble length acoustic line, which comprises terminating the output end of the line With the material under investigation, impressing a sound wave on the input end of the line, varying the length of the line, and measuring the ratio of the maximum to minimum impedance varied.

4. The method of determining the amplitude coefficient of reflection, C of acoustic material by means of a uniform acoustic line, which comprises closing the output end of the line by the material under consideration, impressing a sound Wave on the input end, varying the length of the line, and measuring the input impedance for various lengths to obtain the ratio, A, of the maximum input impedance to the minimum input impedance from which ratio the amplitude coefiicient of reflection may be calculated by the formula,

pressing sound waves on the atmosphere within the tube, means for closing the other end at the input endas the length is i of the tube with the material under consideration, and means for measuring the impedance of the tube at said first mentioned end.

6. Apparatus for measuring th acoustic properties of porous materials comprising a tube, means 'at one end of the tube for impressing sound waves on the atmosphere within said tube, variable means for placing the material under consideration at a chosen distance from said first mentioned means so as to close the tube at that point,

and means for measuring the ratio of the impedances of the tube at said'first mentioned end for,various positions of-the material.

7. Apparatus according to claim 6, in which the wave impressing means has a velocity substantially independent of the impedance into which it works and the impedance ratio measuring means comprises a device measuring the .pressure of the atmosphere within the tube at the first mentioned end.

8. Apparatus :for measuring the acoustic properties of porous materials comprising a tube containing a sound conducting medium, a plunger of large mass, means of negligible stiffness for l'nonnting said plunger in one 'end of the tube, means for impressing vibratory motion onsaid plunger, a hollow piston for closing said tube, said piston being arranged to hold the materialunder investiation, and means for measuring the ratio of theimpedances of the tube at the face of said plunger for various positions of the piston. I

9. Apparatus for measuring the properties of acoustic materials-comprising a tube containin a sound conductingmedium, aplunger of arge mass, means of negligible stillness for mounting said plunger in one end of the tube to substantially close it, mean's'for impressing vibratory motion on said plunger,

ahollow piston for closing said tube at a point remote from'said end, said piston bemg arranged to hold the material under con- S1 eration, a small tube having one end extending within said first mentioned tube to a point close to said plunger, and means associated with the other end ofsaid small tube to indicate the pressure at the face of said plunger.

' 10. Apparatus according to claim 9 in which the means for indicating'the pressure comprises means for converting the-sound waves into electrical Waves, and means for indicating the amplitude of said electrical waves;

11. A paratus according to claim 9 in which t e means for indicating-the pressure at the face-of the plunger comprises a condenser transmitter for converting the sound waves into electrical waves.

In witness whereof, I hereunto subscribe my name this 12th da of July, A. D. 1926.

ED YARD c. WENTE.

for 

